An ultrasonic flow meter of the type that the generated ultrasonic wave is caused to propagete through the fluid flowing through the tublar path, the velocity of the fluid flowing through the tublar path is determined on the basis of the difference between the rate of propagation of the ultrasonic wave propagates from the upstream to the downstream and that propagates from the downstream to the upstream, and provide the flow rate of the fluid flowing through the tublar path, is referred to as an ultrasonic flow meter of the type determining the flow rate from the difference between the rates of propagation, or an ultrasonic flow meter of the type determining the flow rate from the difference between the propagation times. Such type of ultrasonic flow meter is accepted broadly in the art.
The above mentioned type of flow meter can be categorized in its structure to two generic types of flow meter.
The first type of flow meter has a straight propagation tube and a pair of ultrasonic oscillators disposed at longitudinally spaced and diametrically opposite positions on the tube so as to face each other, as shown in FIG. 6. The generated ultrasonic wave propagates obliquely to the flowing direction of the fluid fluid flowing through the tube. The ultrasonic flow meter of this type will be referred to hereinafter as "flow meter of type 1".
The second type of flow meter has a detector comprising a straight propagation tube having ultrasonic oscillators at both of its closed ends, and inflow and outflow tubes connected to the side surface of the propagation tube, as shown in FIGS. 7, 8a, and 9. The ultrasonic wave generated by means of ultrasonic oscillator will propagate in parallel with the fluid flowing through the tube. The ultrasonic flow meter of this type will be referred to hereinafter as "flow meter of type 2".
The operating priciple adopted in both of the type 1 and the type 2 is referred to as alternately operating method for measuring the difference between the rates of propagation of the ultrasonic wave. This method is adopted because of its measuring accuracy and the cost for it.
In this method, two ultrasonic oscillators are arranged so that the one of the oscillators may receive the ultrasonic wave generated by another oscillator, and vice versa. In other words, when one of the oscillators functions as an ultrasonic oscillator, the other of the oscillators functions as a receiver for the ultrasonic wave generated by means of the one oscillator, and vice versa. The flow rate can be determined by measuring the difference between the time required for propagating the ultrasonic wave from the one oscillator to the other oscillator and that from the other oscillator to the one oscillator. This difference will be referred to hereinbelow as "propagation time lag".
Each of these flow meters of the type 1 and type 2 has respective advantages and disadvantages described hereinbelow, so that these flow meters will be utilized properly for the application.
The primary feature of the flow meter of type 1 is that the components of the flow meter such as the inflow tube, the propagation tube, and the outflow tube are included in a continuous straight tube. In this connection, the flow meter of type 1 has a number of advantages such that the flow meter can be manufactured economically, the pressure loss through the flow meter is very little, the cleaning of the flow meter can easily be carried out, and so on.
However, the smaller the diameter of the tube, the smaller the distance through which the ultrasonic wave propagates, and the mesuring accuracy of the propagation time lag will be decreased accordingly. In this connection, the flow meter of type 1 is not suitable for the tubings of smaller diameter (i.e. the tubings of lower flow rate). Actually, a diameter of the tubings of the diameter in the order of 25 mm is a lower limit for the flow meter of type 1. The flow meter of type 1 will demonstrate its advantages when applied to the tubings of larger diameter.
On the other hand, the flow meter of type 2 is complex in its structure, and inferior to the type 1 in its weight, size, manufacturing cost, as well as pressure loss and cleaning ability. However, the flow meter of type 2 is suitable for the tubings of smaller diameter (i.e. the tubings of lower flow rate), because the propagation distance of the ultrasonic wave can be set irrespective of the diameter of the tubing.
In summary, the flow meter of type 1 is for the larger flow rate, whereas the flow meter of type 2 is for the smaller flow rate.
In the case of the prior flow meter of type 2 for the smaller diameter, it is necessary to set the distance between two oscillators larger than a certain distance for assuring predetermined accuracy of the flow meter. Further, the oscillatory energy larger than the predetermined value is required for propagating the detectable ultrasonic wave between the oscillators. It is, therefore, impossible to vary the length L of the propagation tube and the diameter D of the oscillator.
Although the diameter of the propagation tube can be reduced, if the diameter of the flow meter shown in FIG. 8a be reduced to the condition as shown in FIG. 8b, the percentage of the ultrasonic wave propagating through the fluid is decreased in accordance with the reduction of the diameter of the propagation tube, so that the measurement will be difficult.
The lower limit of the inner diameter of the tubing of the practically used flow meter of type 2 is in the order of 5 mm.
It is evident from the above description that the most important improvement to be made on the ultrasonic flow meter is to simplify the structure of the flow meter as the flow meter of type 1, and to make the flow meter applicable to the tubings of the smaller diameter.